Recently, the heterostructure of silicon and silicon germanium is used for the higher performance of the MOS transistor using silicon.
As one technique of improving the performance of the NMOS transistor, it is known to form a tensile-strained silicon layer on a lattice-relaxed silicon germanium layer to use the silicon layer as the channel. As methods of forming a lattice-relaxed silicon germanium layer are known the method of forming a silicon germanium layer sufficiently thick, the method of forming a silicon germanium layer relatively thin on a silicon layer formed on an insulation film and lattice-relaxing the silicon germanium layer by thermal processing, and other methods. As one technique for improving the performance of the PMOS transistor is known the method of forming a compression-strained silicon germanium layer on a silicon substrate to use the silicon germanium layer as the channel.
Furthermore, as transistors are increasingly downsized, the parasitic capacitance decrease and the short channel effect prevention are more required. As a method for meeting these requirements, the SOI (Silicon On Insulator) structure, in which a channel layer, etc. are provided on a silicon layer on an insulation film, is noted.
Conventionally, a tensile-strained silicon layer has been formed on the SOI structure as follows.
First, a compression-strained silicon germanium layer is formed on a silicon substrate. Then, oxygen is implanted into the silicon substrate by SIMOX (Separation by Implanted Oxygen) to form an insulation layer of silicon oxide film below the silicon germanium layer. Then, the compression-strained silicon germanium layer is lattice-relaxed by high temperature thermal processing at, e.g., 1200° C. Then, a tensile-strained silicon layer is formed on the lattice-relaxed silicon germanium layer.
Otherwise, a compression-strained silicon germanium layer is formed on an SOI substrate with a silicon layer formed on a silicon substrate with a silicon oxide film formed therebetween. Next, the surface of the lattice-relaxed silicon germanium layer is oxidized by thermal oxidation and removed, and then a tensile-strained silicon layer is formed on the lattice-relaxed silicon germanium layer.
For the low electric power consumption, it is necessary to combine an NMOS transistor and a PMOS transistor to thereby form an integrated transistor. However, in the MOS transistor using the above-described strained silicon layer or silicon germanium layer as the channel, the strain states required for the NMOS transistor and the PMOS transistor are different from each other. Accordingly, it is difficult to integrate the NMOS transistor and the PMOS transistor on one and the same substrate.
As a method for integrating on one and the same substrate an NMOS transistor using a tensile-strained silicon layer as the channel and a PMOS transistor using a compression-strained silicon germanium layer as the channel is known the method disclosed in, e.g., Patent Reference 1 (Japanese Patent Application Unexamined Publication No. Hei 9-219524).
In the method disclosed in Patent Reference 1, a lattice-relaxed silicon germanium layer as the base layer for forming a tensile-strained silicon layer used as the channel of the NMOS transistor, and a compression-strained silicon germanium layer used as the channel of the PMOS transistor are formed as follows.
First, in a region of an SOI substrate having a silicon layer formed on a silicon substrate with an insulation layer formed therebetween, where the PMOS transistor is to be formed, an opening is formed down to the silicon substrate.
Then, a silicon germanium layer is formed by epitaxial process on the entire surface of the SOI substrate with the opening formed in.
Then, the silicon germanium layer formed on the silicon layer of the SOI substrate is lattice-relaxed by thermal processing. At this time, the thickness of the silicon germanium layer is below a critical film thickness determined by a germanium composition ratio and deposition temperature, whereby the silicon germanium layer formed on the silicon substrate exposed in the opening can be compression-strained.
As described above, in the method disclosed in Patent Reference 1, the silicon germanium layer formed on the SOI substrate is lattice-relaxed in the region where the NMOS transistor is to be formed and compression-strained in the region where the PMOS transistor is to be formed. To this end, it is necessary to form in advance the opening down the silicon substrate in the region where the PMOS transistor is to be formed. Because of no insulation layer below the compression-strained silicon germanium layer in the region where the PMOS transistor is to be formed, resultant disadvantages will be as follows.
First, because of no insulation film between the compression-strained silicon germanium layer to be used as the channel of the PMOS transistor and the silicon substrate, effects of decreasing the capacitance, etc., which are characteristic of the SOI structure, is deteriorated.
Because of no insulation film below the silicon germanium layer in the region where the PMOS transistor is to be formed, a large step is formed between the region where the NMOS transistor is to be formed and the region where the PMOS transistor is to be formed. Accordingly, the flatness of the surface of the substrate cannot be ensured, which will make it difficult to prevent the decrease of the processing precision.
Furthermore, to the conventional method using the above-described strained semiconductor layers, the technique for controlling the strain of the semiconductor layers based on the once formed and strained silicon germanium layer is very important. However, in order to lattice-relax the compression-strained silicon germanium layer, high-temperature and long-time thermal processing has been so far required. Accordingly, even the semiconductor layer, etc. which require no heating are heated, and resultantly this thermal processing has often generated defects, changed impurity profiles, etc., which have decided the device characteristics.
An object of the present invention is to provide a semiconductor device having a tensile-strained silicon layer and a compression-strained silicon germanium layer formed in good alignment with each other on one and the same substrate, and a method for fabricating the same.
Another object of the present invention is to provide a method for fabricating a semiconductor device which allows a strain of the silicon germanium layer to be controlled selectively in a short period of time, and a method for fabricating the semiconductor device.